Topic 14: Consciousness Flashcards
What is the biological basis of consciousness?
natural evolution, as brilliantly by Charles Darwin (1809-1882), over these many million years gave rise to nervous systems as complex as the human brain, arguably the most complex object in the universe
and somehow, through the interactions among its 100 billion neurons, connected by trillions of synapses, emerges our conscious experience of the world and ourselves
What is the “hard problem of consciousness” proposed by Chalmers (1995)?
is the problem of explaining the relationship between physical phenomena, such as brain processes, and experience (i.e., phenomenal consciousness, or mental states/events with phenomenal qualities or qualia)
why are physical processes ever accompanied by experience?
and why does a given physical process generate the specific experience it does - why an experience of red rather than green, for example?
What is the “easy problem of consciousness”?
typically involves pursuing a reductionist approach, such as looking for correlational relationships between brain states and experience (e.g. mind wandering and the default network) that may amount to claims about functionality (e.g. the default network is “for” mind wandering, the visual cortex is “for” seeing, etc.)
What are three general methodological approaches to studying consciousness?
physiological
content-based
theoretical
What are some common examples of altered states of consciousness?
sleep
anesthesia
psychoactive drugs
meditation
various clinical states (e.g. coma)
What is a coma?
various causes of brainstem injury (stroke, CNS infection, hypothermia, metabolic disorder, etc.) can lead to coma
a coma can be described as “… a state of unarousable unconsciousness, characterized by a failure of the arousal/alerting system of the brain (the ascending reticular activating system)
patients in comas do not open their eyes, and only produce minimal motor/auditory responses
What brain regions are involved in comas?
the ascending reticular activating system involves a group of nuclei in the brainstem, including two particularly important ones in the pons that deliver acetylcholine and glutamate to the thalamus
these neurons fire during wakefulness and REM sleep (and GABAergic neurons in the hypothalamus and basal forebrain fire when those other neurons are not, i.e. demonstrating an antagonistic relationship)
comas can therefore be understood as (in part) a failure of this system (the ascending reticular activating system)
based on studying coma patients, it seems that activity in the reticular activating system is necessary for consciousness
What is a vegetative state or unresponsiveness wakefulness syndrome (UWS)?
vegetative patients have their eyes wide open but are considered to be unaware of themselves or their surroundings
they may grimace, cry or smile (albeit never contingent upon specific external stimuli) and move their eyes, head and limbs in a meaningless “automatic” manner
the vegetative state is often, but not always, chronic (the “persistent vegetative state”)
given proper medical care (i.e. artificial hydration and nutrition) patients can survive for many years
What are some examples of various diagnostic criteria for vegetative state/UWS?
no meaningful response to external stimuli
no bladder/bowel control
no sustained visual-pursuit eye movements
no volitional fixation
no purposeful limb movement
~50% global brain metabolic activity relative to baseline (similar to nonREM sleep or anesthesia)
Is recovery possible from a coma or vegetative state?
patients sometimes recover from comas/vegetative state/UWS on their own (without intervention), though there have also been various experimental attempts to expedite/enhance that process
What is locked-in syndrome (LIS)?
similar in some ways to a vegetative state/UWS
a state in which selective supranuclear motor deafferentation produces paralysis of all four limbs and the last cranial nerves without interfering with consciousness
the voluntary motor paralysis prevents the subjects from communicating by word or body movement
usually, but not always, the anatomy of the responsible lesion in the brainstem is such that locked-in patients are left with the capacity to use vertical eye movements and blinking to communicate their awareness of internal and external stimuli
often caused by a stroke in the ventral pons
What is the average time course of diagnosis for LIS?
average time-course of diagnosis for LIS (from injury): 78 days
according to one survey, family members made the original observation that a patient may actually be conscious in 55% of cases (compared to 23% with doctors)
extreme methods of communicating possible (Bauby (1997) dictated his book letter-by-letter, blinking each time his therapist spoke the desired letter from a frequency-ordered list)
What physiological/neurological changes occur when a coma/vegetative state/UWS patient recovers?
while UWS is typically associated with global metabolic changes (e.g., 50% of normal levels), the relationship between abnormal metabolic activity and consciousness is not always straightforward
measurement of metabolic activity alone would therefore seem to lack the sensitivity/specificity to use as a definitive way to categorize/understand what’s happening in the brain of these patients
e.g. some patients recover from UWS without their metabolism returning to normal, and some neurologically healthy patients have “abnormal” metabolism without any accompanying changes in their level of consciousness
Why is measuring connectivity a useful diagnostic to measuring metabolic changes to evaluate consciousness?
using PET, one case study patient was found to have
reduced functional connectivity (relative to a control group) between their thalamus and regions of the lateral PFC and anterior cingulate cortex two weeks after presenting with UWS
restored (“normal”) connectivity in those regions four months after recovery
consistent with the implication that abnormal connectivity is associated with UWS, other UWS patients who fail to recover tend to show persistent abnormal connectivity (within their cortex, as well as between their cortex and thalamus)
What was the Owen et al. (2006) study on detecting awareness?
23 year-old TBI patient diagnosed as being in a vegetative state
Owen et al. (2006) reported activity in certain regions (STG and MTG) was greater for speech vs. noise)
patient was also asked to imagine either:
1. playing tennis
2. walking around their house, visiting each room
What was the Monti et al. (2010) study on detecting awareness?
out of a total of 54 patients (with disorders of consciousness) who were scanned, they found 5 who could modulate neural activity on command
also used the paradigm in Owen et al. (2006) to serve as a yes/no response to a series of autobiographical questions (e.g. if answer to my question is yes, imagine playing tennis)
What are two relevant dimensions for understanding/predicting consciousness?
level of physiological arousal of the brain (which is necessary but not sufficient)
level of awareness (currently ill defined)
What physiological changes occur when we fall asleep?
arousing signals sent to the thalamus and cortex (from the brainstem and basal forebrain) are inhibited
What do the physiological changes occur when we fall asleep result in?
a reduction in connectivity between brain regions
an increase in the activity of certain cortical neurons whose waking activity is not usually correlated
alpha waves are suppressed, theta waves emerge, then delta waves
What physiological changes occur during REM sleep?
characteristics similar to wakefulness
release of ACh (acetylcholine) and glutamate (but not other neurotransmitters that are normally involved in wakefulness: serotonin, norepinephrine, etc.)
a higher proportion of time spent dreaming
what we might consider to be “sleep consciousness” (which may provide another opportunity to study consciousness)
How is simultaneous TMS and EEG used to study sleep?
TMS and EEF can be used in conjunction to assess connectivity, which can be compared during different states of consciousness
e.g. deliver a burst of electrical stimulation with TMS (essentially acting as input to the system), then record the subsequent electrical response of the system via EEG (essentially acting as output for the system)
similar to idea of presenting stimuli and recording the neural response, yet more direct
What was the Massimini et al. (2005) study on simultaneous TMS and EEG during sleep?
TMS-evoked response during nonREM sleep (as compared to wakefulness) produced:
1. a larger initial magnitude of response
2. a response that lasted for a shorter duration, and seemed to affect less widespread regions
the second effect attributed to a reduction in connectivity during nonREM sleep, resulting in the signal remaining “local” (i.e. did not activate a series of other areas)
What are the implications of the Massimini et al. (2005) study on simultaneous TMS and EEG during sleep?
what about the first effect (produced a larger initial magnitude of response)?
seems counter-intuitive, why would we have a stronger response while in a less aware state?
the authors suggest this may relate to the fact that nearby neurons can be more tightly synchronized during nonREM sleep, meaning the response to a directly stimulated area was also more synchronized (which could result in a stronger response)
What were the methods in the Voss et al. (2009) EEG study on lucid dreaming?
required 20 participants to take part in weekly training sessions aimed at helping them experience lucid dreams
after 4 months of training, 4 participants reported reliable being able to successfully induce lucid dreaming (average 3 times/week)
these participants were the brought into a sleep lab and EEG recording were taken while they slept
What were the results in the Voss et al. (2009) EEG study on lucid dreaming?
among their results were data showing coherence levels during lucid dreaming were similar to during wakefulness (both of which were higher than during REM sleep)
other aspects of the EEG data recorded during lucid dreaming resembled REM, with some other differences (e.g., higher power in the gamma band)
EOG (related to eye-movements) and EMG (related to muscle tone/activity) signals recorded during lucid dreaming were immediate between REM and wakefulness
What are the methodological challenges to studying lucid dreaming?
small sample sizes
trial and error nature of “catching” a lucid dreaming episode in the lab with EEG, fMRI, etc.
question of how to operationalize lucid dreaming
What are the differences between REM sleep and nonREM sleep?
relative to non-REM sleep, REM sleep is associated with:
increased activation in: higher visual areas, the amygdala, medial prefrontal cortex, anterior cingulate cortex
decreased activation in: anterior prefrontal cortex, various parts of parietal cortex
What were the results of the Dresler et al. (2012) study on lucid dreaming?
as compared to non-lucid REM sleep, during lucid dreaming in their case study, Dresler et al. (2012) observed greater activation in several areas, including prefrontal and occipito-temporal cortices
perhaps not surprisingly, increased activation was also found in the superior frontal gyrus, which has been associated with self-awareness in other research
How does lucid dreaming fit on the two dimensions of awareness and physiological arousal?
we could argue that lucid dreams represent a state in which the level of both awareness and physiological arousal are intermediate between non-lucid dreaming and waking consciousness
What is anesthesia?
a chemically induced (and reversible) loss of sensation
What is general anesthesia?
causes a dose dependent decline in overall brain glucose metabolism
seems to involve a breakdown of effective connectivity
What is functional connectivity?
refers to correlated activity between two regions
What is effective connectivity?
refers to the ability of one region exerting a causal influence over another
What was the study by Ferrarelli et al. on using simultaneous TMS and EEG on anesthesia?
emulated the general approach taken in Massimini et al. (2005) but extended it to the effects of sedation (using Midazolam, a benzodiazapene)
Benzos are GABA agonists
similar to the effects while sleeping, TMS-evoked responses during sedation were of shorter duration (as compared to during wakefulness)
could be interpreted as reflecting a reductio in effective connectivity
still only correlational, though suggestive that coordinated activity across the brain may be important/necessary for a “normal” conscious experience
How was surface and deep brain EEG used to dissociate cortical and subcortical activity?
patients with Parkinson’s disease with electrodes implanted in the subthalamic nucleus of the basal ganglia
provide subcortical field potential recordings (ESCoG: subcortical EEG)
subcortical EEG provides measurements of electrical activity associated with regions EEG wouldn’t typically pick up (because their location/placement “deeper” within the brain)
the signal from (both types of) EEG can be used to calculate a measurement of complexity (“dimensional activation” or D)
higher/lower D = supposed to reflect greater/less “complexity”
higher/lower D = also correlates with higher/lower levels of alertness
What does the electrical activity of the cortical and subcortical structures look like after anesthesia (propofol) induced loss of consciousness?
subcortical: gradual reduction in D
cortical: sudden drop in D (at the point where consciousness is lost)
“all or nothing” pattern suggests changes in cortical activity seem more closely coupled with the subjective experience of consciousness
How can simultaneous TMS and EEG be used in coma patients?
the TMS EEG procedure can be used to produce measurements at different time points to track any changes in patients
coma patients who do recover show a relatively clear pattern of change in their neural response to the TMD pulses
in contrast, patients who do not recover do not show this pattern
How could we test whether V1 activity contributes to the subjective experience of consciousness?
two general approaches each have the ability to potentially dissociate what is happening at the retinal level with the rest of the brain
- could use binocular rivalry: a paradigm involving presenting different images to each eye, which tends to result in conscious awareness/perception of just one at any give time
- can also present a single item on each trial that is difficult to perceive (e.g. because of brief stimulus presentation times combined with masking) and compare trials in which it is detected to those in which it is not
What are the four general possibilities for combinations of stimuli and responses in signal detection theory?
correct responses: correct rejections and hits
incorrect responses: false alarms and misses
What was the Less and Heeger (2003) fMRI study on activity in early sensory cortex?
used fMRI to measure the BOLD signal in V1 (comparing trials in which items are seen to those in which they are not)
it was assumed that…
correct rejection: can be considered a baseline
hit: will produce an elevated response (relative to baseline)
incorrect responses: less clear what to predict for…
false alarm: item was perceived but not there (!)
miss: item was there but not perceived
What are the two general possibilities for what happens in V1 when a target is presented but we do not perceive it (i.e. a miss)?
it could be the case that V1 codes for the “true” state of whatever stimuli are actually out there in the environment, regardless of whether they reach conscious awareness (which could depend on processing in “higher-level” areas, not V1 activity)
it could also be the case that activity in V1 is more closely coupled to your conscious experience (in other words, will demonstrate a patter of activity that is consistent with whatever you are consciously aware of, not necessarily what is actually out there in the environment)
What were the results of the Less and Heeger (2003) fMRI study on activity in early sensory cortex?
correct rejections: the dashed line (the baseline)
hits: produces an elevated response (as expected)
but what does activity look like during misses and false alarms?
in either case you could be said to have incorrect information about whether a target is present, yet they differ in whether the target is actually there
misses look a lot like correct rejections
false alarms look like hits, though with levels of activation intermediate between hits ad the other two types of trials (correct rejections and misses)
What is the relationship between masking, priming, and repetition suppression and reading with and without awareness?
words presented briefly while (backward) masked are typically not consciously perceived, yet still produce priming effects
this means they are still being processed on some level, yet (for whatever reason) that processing does not seem to reach conscious awareness
What were the results of the Dehaene et al. (2001) study on repetition priming?
confirmed repetition priming in their fMRI study
converging neurological evidence was also found that demonstrates processing on some level, albeit not one that reached conscious awareness
this came in the form of repetition suppression (a diminished BOLD response to a reoccurring stimulus) in the left fusiform gyrus (sometimes considered the “visual word form area”
unmasked words also activated a more widespread area of cortex (masked words only activated two regions: left fusiform gyrus and left precentral sulcus)
What is the global workspace theory of consciousness?
according to the global workplace theory, our consciousness can be though of a bit like the desktop on a computer, which involves multiple “programs” running in the background that are all more or less immediately accessible
these multiple “programs” often serve one of three function: input, output, or computations
stimuli compete for representation in the global workspace
“all or nothing” property of conscious awareness: we are either aware of something or not, in a binary way
in some instances, this may be a good description of what is happening with the attentional blink
What is the recurrent processing theory of consciousness?
emphasizes the role played by feedback signals (in contrast to just “feedforward” signals)
the inclusion of both feedback and feedforward type signals may help facilitate widespread coordination of activity throughout the brain
